1. Field of the Invention
The present invention relates to a developing device which is used in an image forming apparatus such as a copying machine, a printer, or the like, and which develops an electrostatic latent image on an image carrier.
2. Related Background Art
As conventional electrophotography methods, a large number of methods are known, as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748, and the like. In general, an electrical latent image is formed on a photosensitive body consisting of a photoconductive material, by one of various means, a toner image is formed using a toner, the toner image is transferred onto a transfer medium such as a paper sheet, as needed, and the toner image transferred onto the transfer medium is fixed by heat, a vapor of a solvent, or the like, thus obtaining a copy. Also, various methods of visualizing an electrical latent image using a toner are known.
As developing methods, a large number of developing methods, for example, a magnetic brush developing method described in U.S. Pat. No. 2,874,063, a powder cloud method described in U.S. Pat. No. 2,221,776, a fur brush method, a liquid developing method, and the like, are known.
Of these developing methods, a magnetic brush method, a cascade method, a liquid developing method, and the like, which use a developing agent mainly consisting of a toner and a carrier, have widely been put into practice. These methods are excellent in that they can relatively stably obtain satisfactory images, but have common drawbacks associated with a two-component developing agent, i.e., degradation of the carrier and a variation in mixing ratio of the toner and carrier.
In order to avoid such drawbacks, various developing methods using a one-component developing agent consisting of only a toner have been proposed. For example, U.S. Pat. No. 3,909,258 proposes a method of developing an image using a magnetic toner having conductivity. In this method, a conductive magnetic toner is supported on a cylindrical conductive sleeve having a magnetism therein, and an electrostatic latent image is developed by bringing the sleeve into contact with the latent image. In this case, a conductive path is formed between the surface of a recording medium and the surface of the sleeve by toner particles on a developing region, an electric charge is guided from the sleeve to the toner particles via the conductive path, and the toner particles become attached to an image portion by a Coulomb force with the electrostatic latent image portion, thereby developing the image. The developing method using a conductive magnetic toner is an excellent method since it can avoid conventional problems associated with a two-component toner. However, since the toner is conductive, it is difficult to electrostatically transfer the developed image from the recording medium to a final support member such as a normal paper sheet.
In order to solve this problem, as a developing method using a high-resistance toner which can be electrostatically transferred, a developing method utilizing dielectric polarization of toner particles is described in Japanese Laid-Open Patent Application No. 52-94140. However, this method has drawbacks such as a low developing speed at which the developed image cannot have a sufficient density, and it is difficult to put this method into practical use. As another method using a high-resistance toner, a method of developing an image by triboelectrification, i.e., electrification of toner particles by friction between toner particles themselves or friction between toner particles and a sleeve, and bringing the charged toner particles into contact with an electrostatic holding member, is known. However, this method has the following drawbacks. That is, the number of times of contact between the toner particles and the friction member is small, and the toner particles are often insufficiently triboelectrified. In addition, when the Coulomb force between the charged toner particles and the sleeve is strong, the toner particles easily cohere. Thus, it is pointed out that many practical problems remain unsolved.
Japanese Laid-Open Patent Application No. 54-43036 proposes a novel developing method which can eliminate the above-mentioned drawbacks. In this method, a very thin toner layer is coated on a developing sleeve, is triboelectrified, and is brought very close to an electrostatic latent image under the influence of a magnetic field to face the image without contacting it, thereby developing the image.
According to this method, since a very thin layer of a magnetic toner is coated to increase the number of times of contact between the magnetic toner and the developing sleeve, a triboelectrification electric charge amount required for development can be developed in the toner.
However, even in the above-mentioned method, it is known that the number of times of contact between the developing sleeve and the magnetic toner is smaller than that between the toner and carrier in a two-component type developing agent.
Also, it is well known that the number of times of contact between the magnetic toner and the developing sleeve required for the magnetic toner to acquire a triboelectrification electric charge amount required for development varies depending on the composition of the magnetic toner, and the like.
Therefore, in the above-mentioned method, in a system which requires a larger number of times of contact to acquire a required triboelectrification electric charge amount, a phenomenon caused by charging instability tends to occur.
Furthermore, it is well known that the number of times of contact between the magnetic toner and the developing sleeve required for the magnetic toner to acquire a triboelectrification electric charge amount required for development varies depending on the composition of the magnetic toner, and the like.
Therefore, in the above-mentioned method, in a system which requires a larger number of times of contact to acquire a required triboelectrification electric charge amount, a phenomenon caused by charging instability tends to occur.
The present inventors examined electric charges generated by the one-component developing method, and found that the toner behaved as follows in an electric charge developed portion of the one-component developing method.
In FIG. 1, a developing device 20 comprises a toner container 3 for storing a magnetic one-component toner, a developing sleeve 1a which is arranged in the opening portion of the toner container 3 to be rotatable in the direction of an arrow in FIG. 1, and uses a non-magnetic member, a permanent magnet 1b fixed in the interior of the developing sleeve 1a, a magnetic blade 2 which is fixed to the toner container 3 and uses a magnetic member for regulating the thickness of a toner layer, and a toner convey member 4 arranged in the toner container 3. The magnetic blade 2 is arranged to have a constant distance value W from the developing sleeve 1a. In general, the distance is often set to fall within a range from 100 xcexcm to 1 mm.
In the developing device shown in FIG. 1, a magnetic one-component toner is coated as a thin layer on the developing sleeve 1a. The thickness of the toner layer is determined by the position of a cut line L shown in FIG. 2.
According to the examinations of the present inventors, it was found that an electric charge was developed in a magnetic toner T when the magnetic toner T passed between the developing sleeve 1a and the magnetic blade 2. It was also found that the magnetic toner behaved as follows.
As shown in FIG. 3, planes perpendicular to a line connecting the developing sleeve 1a and the magnetic blade 2 are assumed, the plane closer to the magnetic blade 2 is represented by S1, and the plane closer to the developing sleeve 1a is represented by S2. In general, since the width of the magnetic blade 2 is set to be smaller than that of the permanent magnet 1b, the magnetic flux densities on the planes S1 and S2 are set, so that the magnetic flux density on the plane S1 becomes larger than that on the plane S2. Therefore, the magnetic toner T receives a force in the direction of an arrow in FIG. 3, i.e., a force toward the magnetic blade 2 side, between the developing sleeve 1a and the magnetic blade 2.
Therefore, as shown in FIG. 2, magnetic toner particles T form ears (state B), and these ears are formed from the magnetic blade 2 in the direction of the magnetic sleeve 1a. The magnetic toner T is charged as follows. That is, when the developing sleeve 1a contacts a toner particle t1 at the distal end of the ear formed from the magnetic blade 2, an electric charge is developed in the distal end.
Furthermore, it was found that the toner was conveyed as follows between the developing sleeve 1a and the magnetic blade 2.
As described above, since an electric charge is developed in the toner particle t1 at the distal end of the ear which contacts the developing sleeve 1a, a force in the direction of the developing sleeve 1a based on a reflection force acts on the toner particle t1, and a convey force in the rotational direction of the developing sleeve 1a acts on the toner particle t1 by a frictional force with the developing sleeve 1a.
Since a given cohesive force acts between toner particles, a convey force also acts on a toner particle t2, which is in contact with the toner particle t1, via the cohesive force. Furthermore, a convey force via the cohesive force similarly acts on a toner particle t3 in an upper layer portion.
However, the magnetic force in the direction of the magnetic blade 2 acts on the toner between the developing sleeve 1a and the magnetic blade 2, as described above. Therefore, the toner ear is torn off at a position where the convey force acting on the toner overcomes the magnetic force, i.e., at the position of the cut line L in FIG. 2, and the toner particles remaining on the developing sleeve 1a are conveyed in the rotational direction of the developing sleeve 1a.
Therefore, as is apparent from the toner behavior and a process for developing an electric charge in the toner in the example using the magnetic blade, an electric charge can only be developed in toner particles near the developing sleeve 1a.
In the above-mentioned example, toner particles in which an electric charge is not developed conglomerate, as indicated by C in FIG. 2, and if this conglomerate of toner particles becomes large, the magnetic force continuously holding the toner particles on the magnetic blade 2 side weakens. For this reason, some toner particles in the conglomerated toner particles are conveyed in the rotational direction of the developing sleeve 1a. As a result, some toner particles conveyed on the sleeve 1a do not develop electric charges, and a phenomenon caused by charging instability tends to occur in the prior art.
In order to solve the above-mentioned problem, the present inventors proposed a developing device which can stably develop an electric charge in a magnetic toner by returning insufficiently charged magnetic toner particles into a developing container by a developing agent regulating member in Ser. No. 250,682.
More specifically, as shown in FIG. 4, as a member for regulating a toner layer coated on the developing sleeve 1a, a developing agent regulating member 6a, which consists of a non-magnetic member, is rotatably arranged near the developing sleeve 1a to face it. In the developing agent regulating member 6a, a permanent magnet 6b is arranged near a magnetic pole N11 of the permanent magnet 1b. Furthermore, a magnetic pole S61 of the permanent magnet 6b in the developing agent regulating member 6a faces the magnetic pole N11 of the permanent magnet 1b in the developing sleeve 1a, and the rotational direction of the developing agent regulating member 6a is set to be the same as that of the developing sleeve 1a. With this arrangement, magnetic toner particles which do not contact the surface of the developing sleeve 1a can be returned into the developing container, and only sufficiently charged magnetic toner particles can be conveyed, thus stabilizing a charging operation of the toner.
As described above, when the developing agent regulating member 6a, which includes the permanent magnet 6b therein, is rotatably arranged near the developing sleeve 1a, the charging operation of the magnetic toner can be stabilized.
However, in the above-mentioned proposal, a sufficiently wide latitude in the arrangement for coating only sufficiently charged magnetic toner particles onto the surface of the developing sleeve cannot often be assured depending on the combination of the magnetic pole N11 in the developing sleeve 1a and the magnetic pole S61 in the developing agent regulating member 6a, cohesion of magnetic toner particles, and the like.
Depending on the combination of the magnetic pole N11 in the developing sleeve 1a and the magnetic pole S61 in the developing agent regulating member 6a, or when the developing agent regulating member 6a consists of a magnetic member, and the width of the member 6a is set to be smaller than the width of the magnetic pole N11, if the rigidity of the developing agent regulating member 6a is low, then the developing agent regulating member 6a flexes by a magnetic force, resulting in coating nonuniformity of a toner layer.
It is an object of the present invention to provide a developing device which can convey only sufficiently charged toner particles to a developing region.
It is another object of the present invention to provide a developing device which can prevent flexure of a regulating member for regulating the toner amount due to a magnetic force.
It is still another object of the present invention to provide a developing device comprising: a toner carrier which moves while carrying a toner on a surface thereof; and a regulating member for regulating an amount of toner on the toner carrier by applying a moving force to the toner in a direction opposite to a moving direction of the toner carrier, wherein in a regulating portion defined by the regulating member, a moving force received by toner which is not in contact with the surface of the toner carrier, from the regulating member, is larger than a convey force received from the toner carrier.
It is still another object of the present invention to provide a developing device comprising: a toner carrier which faces an image carrier for carrying an electrostatic latent image, and which rotates while carrying a toner on a surface thereof; and a regulating rotary member for regulating an amount of toner on the toner carrier by rotation, wherein a regulating portion of the regulating rotary member is arranged on a side of the regulating member opposite the image carrier with respect to a line in a direction of gravity which passes through the center of rotation of the toner carrier.
It is still another object of the present invention to provide a developing device comprising: a toner carrier, having a magnet therein, for carrying a magnetic toner; a regulating rotary member, having a magnet therein, for regulating an amount of toner on the toner carrier; and magnetic field generating means for applying, to the regulating rotary member, a magnetic force in a direction substantially opposite to a direction of a magnetic force received by the regulating rotary member from the magnet in the toner carrier.
Other objects of the present invention will become apparent from the following description.